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[93.34.88.81]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4792b02ba67sm127870165e9.2.2025.12.07.15.20.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 07 Dec 2025 15:20:19 -0800 (PST) Message-ID: <69360bb3.7b0a0220.46cd2.4675@mx.google.com> X-Google-Original-Message-ID: Date: Mon, 8 Dec 2025 00:20:16 +0100 From: Christian Marangi To: Andy Shevchenko Cc: Andrew Morton , Dan Williams , Jonathan Cameron , Magnus Damm , linux-kernel@vger.kernel.org, "Rob Herring (Arm)" , stable@vger.kernel.org Subject: Re: [PATCH] resource: handle wrong resource_size value on zero start/end resource References: <20251207215359.28895-1-ansuelsmth@gmail.com> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Dec 08, 2025 at 01:12:03AM +0200, Andy Shevchenko wrote: > On Sun, Dec 07, 2025 at 10:53:48PM +0100, Christian Marangi wrote: > > Commit 900730dc4705 ("wifi: ath: Use > > of_reserved_mem_region_to_resource() for "memory-region"") uncovered a > > massive problem with the usage of resource_size() helper. > > > > The reported commit caused a regression with ath11k WiFi firmware > > loading and the change was just a simple replacement of duplicate code > > with a new helper of_reserved_mem_region_to_resource(). > > > > On reworking this, in the commit also a check for the presence of the > > node was replaced with resource_size(&res). This was done following the > > logic that if the node wasn't present then it's expected that also the > > resource_size is zero, mimicking the same if-else logic. > > > > This was also the reason the regression was mostly hard to catch at > > first sight as the rework is correctly done given the assumption on the > > used helpers. > > > > BUT this is actually not the case. On further inspection on > > resource_size() it was found that it NEVER actually returns 0. > > > > Even if the resource value of start and end are 0, the return value of > > resource_size() will ALWAYS be 1, resulting in the broken if-else > > condition ALWAYS going in the first if condition. > > > > This was simply confirmed by reading the resource_size() logic: > > > > return res->end - res->start + 1; > > > > Given the confusion, also other case of such usage were searched in the > > kernel and with great suprise it seems LOTS of place assume > > resource_size() should return zero in the context of the resource start > > and end set to 0. > > > > Quoting for example comments in drivers/vfio/pci/vfio_pci_core.c: > > > > /* > > * The PCI core shouldn't set up a resource with a > > * type but zero size. But there may be bugs that > > * cause us to do that. > > */ > > if (!resource_size(res)) > > goto no_mmap; > > > > It really seems resource_size() was tought with the assumption that > > resource struct was always correctly initialized before calling it and > > never set to zero. > > > > But across the year this got lost and now there are lots of driver that > > assume resource_size() returns 0 if start and end are also 0. > > > > To better handle this and make resource_size() returns correct value in > > such case, add a simple check and return 0 if both resource start and > > resource end are zero. > > Good catch! > > Now, let's unveil which drivers rely on "broken" behaviour... > > ... > > > static inline resource_size_t resource_size(const struct resource *res) > > { > > + if (!res->start && !res->end) > > + return 0; > > I think this breaks or might brake some of the drivers that rely on the proper > calculation. If you supply the start and end for the same (if it's not 0), you > will get 1 and it's _correct_ result (surprise surprise). One of the thing that > may be directly affected (and regress) is the amount of IRQs calculation (which > on some platforms may start from 0). However, in practice I think it's none > nowadays in the upstream kernel. > One common usage of this is with address size. So if start and end is the same, then it's ok to have size 1? > > return res->end - res->start + 1; > > } > > That said, unfortunately, I think, you want to fix drivers one-by-one and this > patch is incorrect as it brings inconsistency to the logic (1 occupied address > whatever unit it has may still be valid resource). > Yep but probably never aligned? I don't think there is an arch in the world that is aligned to 1 byte? > Also a good start is to add test cases and add/update documentation. > I hoped this was simple enough to have the condition. The more articulate and safe change might be to: 1. rename this to __resource_size 2. rename every entry of resource_size to __resource_size 3. introduce a new resource_size commented and with the check 4. Use the new helper where it's actually needed? >From my search there are various place where the condition is like: if (resource_size(&res)) ... And this condition doesn't make any sense since it's always true (I highly suspect these case all fall in what I described) For sure this needs to be discussed and we need to gather more info. > -- > With Best Regards, > Andy Shevchenko > > -- Ansuel